1. Field of the Invention
The present invention relates to noninvasive diagnostic medical devices utilizing ultrasound.
2. Prior Art
In ultrasonic medical devices, ultrasound is beamed into the body by a transmitting transducer and waves reflected from internal matter are detected by the same or a separate receiving transducer.
A first category of ultrasonic medical devices is the Doppler device which detects the difference in frequency of reflected waves as compared to the frequency of reflected waves as compared to the frequency of the transmitted waves. The difference in frequency of a wave before and after its reflection indicates the velocity of the matter from which the wave was reflected. Doppler devices are often used for analyzing blood flow through an artery, for example.
In a continuous wave (CW) Doppler device, the ultrasound is continuously transmitted into the body and the reflected waves are continuously monitored for Doppler shift. Separate transmitting and receiving transducers are required and it is not possible to differentiate between waves reflected at different distances from the transmitting transducer.
In a pulsed Doppler device, short bursts of ultrasound are transmitted at uniform intervals. Reflected waves from a desired depth can be monitored by analyzing the received ultrasound signal the appropriate length of time after a pulse or burst of ultrasound was transmitted. Such a system is referred to as a "range-gated" system. The "range gate" is the short period following the transmission during which the received wave form is monitored. In sophisticated systems, there can be several range gates so that the reflected wave form is monitored for each of several different short periods following the pulse transmission, corresponding to the reflection of transmitted waves at each of several different depths. FIG. 1 illustrates the transmitted pulse of ultrasound, the echo and the range gate spaced a predetermined period of time t.sub.d from the beginning of the transmitted pulse. During the range gate period the echo is sampled or monitored to detect the portion reflected from a desired depth.
In one type of medical Doppler device, the electrical signal representing the reflected wave is demodulated to produce an audio-frequency signal varying in frequency (pitch) depending on the difference in detected velocity (Doppler shift). The demodulated signal is fed to a speaker and the resulting sound is indicative of blood flow, such as pulsing of blood through an artery. An example of such a device is the Versadopp.TM.10 device available from Diagnostic Ultrasound Corporation of Kirkland, Washington.
In other medical Doppler devices, the signal representing the reflected wave is fed to a frequency spectrum analyzer which can actuate a video display. Such a display can show a curve representing amplitude on the vertical axis and frequency on the horizontal axis. For analyzing blood flow, the relative amplitude of each frequency component indicates the proportion of blood flowing at the corresponding velocity. Another type of display indicates frequency on the vertical axis and time on the horizontal axis with amplitude represented by brightness. See, for example, the article titled "VASCULAR ULTRASOUND Ultrasonic Evaluation of the Carotid Bifurcation", by Langlois, Roederer and Strandness, published in Vol. 4, No. 2, of ECHOCARDIOGRAPHY A Review of Cardiovascular Ultrasound (Futura Publishing Company, Inc., 1987).
Medical Doppler devices provide information on the velocity of the matter from which the ultrasound is reflected relative to the transmitting transducer. Information on the direction, i.e., toward or away from the transducer, can be obtained by utilizing "quadrature phase signals" generated by known devices as represented in FIG. 2. As pertinent to the present invention, the received signal is compared with two reference signals that are identical except that one signal is 90 degrees out of phase relative to the other signal. Separate demodulators produce outputs commonly known as "I" and "Q" which also are 90 degrees out of phase. Velocity information can be calculated from a single signal and velocity and relative direction can be detected from the two signals. See Peter Atkinson and John P. Woodcock, Doppler Ultrasound and its Use in Clinical Measurement, (San Francisco: Academic Press, 1982), particularly pages 54 to 74.
Another type of noninvasive ultrasonic medical diagnostic device is the "imaging" ultrasound device. Ultrasonic imaging systems are concerned with the amplitude of the reflected wave as a function of time. The amplitude and time information indicate the location of interfaces between different types of tissue having different impedances to ultrasound waves. In a "motion mode" (M mode) system, range gating is utilized to detect amplitude at different depths which can be represented as the vertical axis on a video display. Brightness indicates amplitude and the horizontal axis represents time so that motion of internal tissue along approximately the line of transmission and reflection is indicated on the display.
In a brightness mode (B mode) imaging system, echoes are monitored along closely adjacent transmission axes such as by sweeping the pulsed transmitting-receiving transducer. The desired result is a two-dimensional cross-sectional image of internal structure such as a tumour or a fetus. See P. N. T. Wells, Biomedical Ultrasonics, (San Francisco: Academic Press, 1977), particularly Chapter 9.
Another pertinent type of medical diagnostic device is the plethysmograph designed to measure expansion and contraction of tissue caused by pulsation of blood through the tissue. Some such devices utilize a fluid-filled cuff encircling a limb and measuring displacement of fluid in the cuff (water-filled) or change in pressure (air-filled). Other such devices measure change in tissue electrical impedance or stretching of tissue (strain gauge) or change in optical properties of the tissue being examined (fluctuations in translucency).
There have been attempts to measure displacement of internal matter by use of medical ultrasound devices, primarily movement of arterial walls, such as described in: Charles F. Olsen, "Doppler Ultrasound: A Technique for Obtaining Arterial Wall Motion Parameters," IEEE Transactions; A. P. G. Hoeks, "On the Development of a Multigate Pulsed Doppler System With Serial Data Processing." Ph.D Thesis University of Limburg, Maastricht, Holland, 1982; and Craig J. Hartley et. al., "Doppler Measurement of Myocardial Thickening With a Single Epicardial Transducer," American Journal of Physiology, 245: 6 (1983), pages H1066-H1072. In general, Doppler techniques are utilized to determine tissue velocity. Velocity as a function of time indicates distance or displacement. An accurate measurement is difficult because of the errors introduced during detection of frequency and the mathematical frequency analysis. More pertinent to the present invention is the system described in L. S. Wilson and D. E. Robinson, "Ultrasonic Measurement of Small Displacements and Deformations of Tissue," Ultrasonic Imaginq, 4 (1982), pages 71 to 82, because in that system there is an attempt to monitor the phase of the reflected ultrasound. to the devices measuring frequency shift, the phase signal is analyzed mathematically by integration to approximate displacement of tissue. Consequently, errors can be introduced both in phase measurement and.